Acoustic windows for use in transmitting or receiving acoustic wave form signals in a liquid environment are known. Traditionally these windows have consisted of a single thickness of a metal such as steel that may optionally have been covered by a rubber containing therein a biologically active substance such as a biocide. These biocide containing rubbers tend to inhibit a build-up of marine life on the surfaces of the window as such build-ups can prove disfunctional to the transmission or reception of acoustic waveforms as well as contributing to disfunctional hydraulic disturbances during propagation through the marine environment of a vessel or other object which might embody such a window.
Typically such windows on an exterior surface have interfaced with a body of free liquid such as an ocean, lake or tank. Such windows, on the interior surface, traditionally have at least partially defined a chamber filled with water or with another liquid. Substantial efforts have been expended to configure such windows to be acoustically "clear", that is producing a desirably low distortion and attenuation of sound wave energy as the sound wave energy is passed through the window, and equally, a desirably low distortion of the angle characterizing the impingement of the sound wave energy against the window.
Such windows have been subject to certain undesirable characteristics. For example, windows made of a rigid material such as steel can generate significant quantities acoustic noise associated with the passage of water over the window and can transmit significant quantities of acoustic noise arising from vibrational frequencies associated with the operation of machinery aboard an object or vessel upon which the window is embodied. Additionally, relatively rigid windows can generate a significant bounce or reflective effect for acoustic waveform energy impinging upon the window surface. Such bounce or reflection can result in a substantial reduction in intensity for signals being transmitted through the window. Where reflection occurs from interior surfaces of the window during transmission of an acoustic waveform from within the chamber defined by the window, spurious or erroneous determinations of the existence or position of an echo could result.
Windows such as sonar domes can be required to transmit acoustic energy having a frequency ranging from about 500 Hz to about 500 kHz. These frequencies correspond to wavelengths of about 3 meters to about 0.003 meters in sea water, with the wavelengths being subject to some variation depending upon the liquid material through which the waveform is being propagated. With traditional domes of metal, or those known in the art of reinforced plastic, where the thickness of the material from which the dome is fabricated deviates substantially from one half wavelength of the acoustical frequency being transmitted through the dome, reduction such as through insertion loss, that is 20 log(P.sub.O /P.sub.T), where P.sub.O is the incident pressure of the wave and P.sub.T is the transmitted pressure, can become unacceptable.
A sonar dome or a sonar window must be built to withstand structurally a particular loading. This construction results in an inherently necessary thickness in the material of construction, where rigid materials are employed. Where this thickness substantially deviates from one half the wavelength being transmitted, an effective blindness to certain acoustic frequencies can result by simple reduction of the waveform energy transmitted across the material thickness.
Relatively small windows or sonar domes have been formed frequently of unreinforced rubbers. Such rubbers are remarkably transparent to the transmission of acoustic frequencies in desired wavelength ranges. These windows formed of unreinforced rubbers, however, are susceptible to deformation from hydrostatic and hydrodynamic forces imposed upon the window by the environment in which it is operated. For example, motion of a submarine embodying such a window through the ocean waters can impose hydrodynamic forces upon a window sufficient to deform the window, possibly even into contact with the sonar array positioned therebeneath. Conversely, simple changes in hydrostatic pressure imposed upon the window by reason of a changing operating depth for a submersible embodying a window can impose hydrostatic forces producing similar distortion effects to the window. This distortion phenomenon is associated principally with a relatively low modulus associated with unreinforced rubber.
Various proposals have been made for reinforcing sonar windows made of rubbers. Conventional techniques such as the introduction of a fabric or cord reinforcement into the rubber have met with somewhat limited success. One drawback to the use of fabric or cording reinforcement has been the inherent inclusion of air in the fabric or cording at the time of incorporation into the rubber for reinforcement purposes. This included air is not readily eliminated and becomes trapped in the resulting window. This included air operates as a substantial barrier or reflector to many sonic frequencies, and impairs materially the functionality of any resulting sonar window or dome.
More recently, sonar windows made employing cis-polybutadiene and including a reinforcement of polyvinyl alcohol fibers therein has found application in making sonar windows having acoustic clarity and utility in a range of 200 kHz to 500 kHz. Cis-polybutadiene is a somewhat difficult elastomer with which to work in forming such windows, however, and is relatively expensive. These windows also have not found significant application in a range of desired sonar frequencies of between about 10 kHz and 100 kHz.
Accordingly, a sonar window having utility in a frequency range of between about 10 kHz and about 100 kHz in an aqueous environment, displaying essentially the sonic transparency of unreinforced rubber, and providing a desirable flexural modulus of between 2,000 and 40,000 psi (13,790-275,800 kPa) could find substantial utility in the manufacture of sonic windows.